RISC-V: Don't fail in riscv_of_parent_hartid() for disabled HARTs

RISC-V: Don't fail in riscv_of_parent_hartid() for disabled HARTs

The riscv_of_processor_hartid() used by riscv_of_parent_hartid() fails
for HARTs disabled in the DT. This results in the following warning
thrown by the RISC-V INTC driver for the E-core on SiFive boards:

[    0.000000] riscv-intc: unable to find hart id for /cpus/cpu@0/interrupt-controller

The riscv_of_parent_hartid() is only expected to read the hartid
from the DT so we directly call of_get_cpu_hwid() instead of calling
riscv_of_processor_hartid().

Fixes: ad635e723e17 ("riscv: cpu: Add 64bit hartid support on RV64")
Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Link: https://lore.kernel.org/r/20231027154254.355853-2-apatel@ventanamicro.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>